Logstash filter changes to support messages from Hadoop

* Refactor of https://gerrit.wikimedia.org/r/#/c/140623
* Incorporated feedback from BDavis to eliminate leading underscores and
  conform with Monolog notions of "type" and "channel"
* Only affects messages where type="Hadoop"

Change-Id: I78e6e2a3a8bafbda76c7f6dec55a69233037f2b6